Cyclohexane conformational degrees

Other substituted cyclohexanes are similar to methylcyclohexane. Two chair conformations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative amounts of the two conformations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane conformations, is related to the degree of branching at the atom connected to the ring. A single... 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

The six-membered rings in these T.S.s are more flexible than the five-membered T.S.—(81) above—and need not be planar (cf cyclohexanes v. cyclopentanes). Elimination may thus proceed, in part at least, from conformations other than the syn-periplanar, with the result that the degree of SYN stereoselectivity in these eliminations may sometimes be lower than that observed in the Cope reaction. Both reactions require higher temperatures than for the Cope reaction, carboxylic esters particularly so. 

Fig. 17. Calculated conformations (top and side views, drawn to scale) of cyclohexane (force field of ref. (79) outer values bond lengths in A and angles in degrees, inner values torsion angles). The quantities below the symmetry symbols are calculated A V- and AH-values (kcal mole-1 T = 298 K reference chair conformation)...

Conformational effects on 15N shifts in substituted cyclohexanes make an axial NH2 more shielded than an equatorial one. Also, 15N resonances are deshielded by ft substitution more extensively than are 13C resonances of cyclic hydrocarbons, but the magnitude of the effect depends on the degree of nitrogen substitution. Carbons in the y position shield the nitrogen in a manner analogous to 13C, but to a smaller extent in methanol than in cyclohexane solutions, and less for tertiary amines than for primary and secondary amines. These differences have been attributed in part to possible conformational influences on the stereoelectronic relationships between the lone pair and the C—C bonds. 

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

Crystallographic evidence amply demonstrates that the trioxane ring is similar in shape and conformational preference to cyclohexane. The barrier for chair-to-chair interconversion, a hitherto unknown quantity, has been determined by a dynamic NMR spectral study <93JCS(P1)1927). The behavior of 3,3,5,5,6,6-hexamethyl-1,2,4-trioxane (27) is typical. Despite the high degree of substitution, MM3 calculations show that it exists as a chair in the ground state. The value determined... 

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